This invention relates to selectively gas permeable membranes and more specifically to a process for making a polymer membrane of a predetermined selectivity with respect to a binary gas combination by blending a separation performance altering substance with the membrane polymer composition.
Separating the components of a gas mixture by contacting the mixture with a membrane that is selectively permeable for the components is a well established and highly valued commercial chemical process unit operation. In such a process, the permeabilities through the membrane of the components to be separated are different. The more preferentially permeable components (i.e., those of higher permeability) permeate faster than the less preferentially permeable components. Hence, the faster permeating components concentrate in the membrane permeate composition.
Through careful design of membrane separation parameters such as membrane composition, gas transfer area, membrane thickness, and equipment configuration (e.g., multiple staging and recycling), and operating conditions such as temperatures, pressures and flow rates, it is often possible to obtain product gases of desired component purity from starting gas mixtures. Selectivity of the membrane for individual components in the mixture is a fundamental if not the primary factor which determines the efficiency of a membrane separation process.
The selectivities of diverse selectively gas permeable membrane candidate compositions have been studied extensively. An important study by L. Robeson (Journal of Membrane Science, 62, pp. 165-185, 1991) correlated selectivity to permeability by plotting the logarithm of separation factor (ie., ratio of permeability of the more preferentially permeable component of a specific gas binary to the permeability of the other gas) versus the logarithm of permeability of the more preferentially permeable component for many polymeric membranes. This correlation is sometimes referred to as a xe2x80x9cRobeson plotxe2x80x9d.
For any given gas binary, for example, He/H2, O2/N2 and CH4/CO2, the performance of each polymeric membrane composition is represented by a single point on the Robeson plot. Robeson plots further demonstrate that for any gas binary there is an inverse linear log-log relationship for the upper bound of separation factor versus permeability through polymer membranes. That is, all the empirically determined data points lie below a negatively sloped line. For example, the limit line of a Robeson plot for the oxygen-nitrogen gas binary is shown as R in FIG. 1.
It was traditionally understood that each membrane composition has but one intrinsic selectivity for a given mixture of gases to be separated under fixed conditions such as temperature and pressure. Moreover, it is not always possible to choose freely among the many known gas permeable polymeric membrane candidate compositions to obtain optimized selectivity versus permeability characteristics. This is because the components of certain gas mixtures can be aggressively reactive with or corrosive to most membrane materials that have desirable separation properties. The separation of chlorine from hydrogen in chlor-alkali processes and of hydrogen fluoride from various gas mixtures in the synthesis of fluorocarbons for the refrigerant industry are typical examples in which the ability of the membrane to resist attack by the gases significantly affects the choice of membrane.
It would thus be desirable to alter the separation characteristics with respect to a given set of gases of a given membrane polymer without changing the basic polymer composition. Such an ability is wanted to provide one membrane type with different selectivity/permeability combinations suited to correspondingly different separation operating conditions. It is also desireable to increase the versatility of highly valued membrane polymers by rendering them useful under a greater range of separation performance requirements. It is especially desirable to adjust the separation characteristics of membrane polymers, such as fluoropolymers, for use in chemically aggressive fluid environments where the selection of alternative membrane materials is limited.
The present invention is directed to separation of components of a gas mixture by permeating the components through a selectively gas permeable nonporous membrane formed from a polymer composition of a uniform blend comprising as a major fraction a fluoropolymer and a nonfugitive, nonpolymeric, fluorinated adjuvant. Accordingly, there is provided a membrane composition for separation of components of a mixture including a more preferentially permeable gas and a less preferentially permeable gas of a binary gas combination comprising a nonporous gas permeable membrane of a blend comprising about 50-99 wt % of a fluoropolymer and an amount of a nonfugitive, nonpolymeric fluorinated adjuvant effective to produce a separation factor of the membrane with respect to the two gases of the binary gas combination greater than the separation factor of a membrane consisting essentially of the fluoropolymer.
There is also provided a gas separation device comprising a selectively gas permeable membrane having a membrane separation factor with respect to two gases, the membrane comprising a uniform blend of a major fraction of a fluoropolymer of which a nonporous membrane exhibits a fluoropolymer separation factor with respect to the two gases, and an amount of a nonfugitive, nonpolymeric fluorinated adjuvant effective to make the membrane separation factor greater than the fluoropolymer separation factor.
This invention further provides a process for making a gas separation membrane comprising the steps of mixing a fluoropolymer comprising a monomer selected from the group consisting of tetrafluoroethylene, chlorotrifluoroethylene, perfluoro-2,2-dimethyl- 1,3 -dioxole, 2,2,4-trifluoro-5-trifluoromethyl- 1,3-dioxole hexafluoropropylene, vinylidene fluoride and a perfluoroalkylvinyl ether, and a nonfugitive, nonpolymeric fluorinated adjuvant to obtain a uniform composition of which the fluoropolymer comprises a major fraction, and fabricating a nonporous gas permeable membrane of about 0.05 - 50 xcexcm thickness from the composition.
Still further there is provided a process for separating gases present in a gas mixture comprising contacting the gas mixture with one side of a selectively gas permeable membrane having a membrane separation factor with respect to two gases in the mixture, the membrane comprising a uniform blend of a major fraction of a fluoropolymer of which a nonporous membrane exhibits a fluoropolymer separation factor with respect to the two gases, and an amount of a nonfugitive, nonpolymeric fluorinated adjuvant having an atmospheric boiling point at least about 200xc2x0 C., the amount being effective to make the membrane separation factor greater than the fluoropolymer separation factor.